1. Field of the Invention
The present invention relates to a liquid crystal composition exhibiting a cholesteric phase at room temperature. The present invention also relates a liquid crystal display element having a light control layer including the liquid crystal composition retained between a pair of substrates at least one of which is transparent. Specifically, the present invention relates to a liquid crystal display element of the reflecting type used for switching between two stable states (bistable switching) by a light control layer including the liquid crystal composition.
2. Description of the Related Art
There have been various researches in recent years into liquid crystal display elements of the reflective type using chiral-nematic liquid crystal expressing a cholesteric phase at room temperature by adding a chiral agent to a nematic liquid crystal.
Heretofore, liquid crystal display elements of the reflective type using a chiral nematic liquid crystal composition have not been able to be driven at low voltage while keeping display characteristics such as reflectivity under the planer state, contrast between the planar state and focal-conic state, color purity (excitation purity), and so on. Furthermore, it is important that this type of liquid crystal display element has a broad operational temperature range (temperature compensation range).
To resolve the previously mentioned disadvantages, a main object of the present invention is to provide an improved liquid crystal composition and reflective type liquid crystal display element using the same.
Another object of the present invention is to provide a liquid crystal composition and a liquid crystal display element of the reflective type having a broad temperature compensation range and a low drive voltage.
The present inventors investigated various compositions of liquid crystal materials to attain these objects, and as a result discovered the efficacy of using a liquid crystal including a liquid crystal ester compound having large dielectric anisotropy for realizing a low drive voltage, and particularly having a CN group at the terminal group and/or a fluorine atom within the molecular structure. The liquid crystal ester compound generally has a high viscosity that is disadvantageous from the perspective of responsiveness. The problem of high viscosity occurs particularly in liquid crystal compounds including more than 10% chiral agent. Furthermore, liquid crystal ester compounds generally have small refractive index anisotropy, and cannot produce adequate contrast. In order to resolve these problems, the inventors of the present invention discovered the advantage of adding liquid crystal tolan compounds having low viscosity and high refractive index anisotropy. Liquid crystal tolan compounds also have high transition temperatures and broad temperature compensation ranges. Furthermore, liquid crystal phenylcyclohexane compound possesses both stability and markedly low viscosity, and it was discovered that adding this compound improved reliability of the entire liquid crystal compound. Compounds having a cyano group as the terminus in particular have been confirmed as increasing the breadth of the temperature compensation range. These studies have resulted in the present invention.
That is, according to one aspect of the present invention, a chiral nematic liquid crystal composition that exhibits a cholesteric phase comprises a nematic liquid crystal composition and a chiral agent having an optical activity. The nematic liquid crystal composition is defined to comprise at least one type of liquid crystal ester compound that includes an ester unit and at least two fluorine atoms therein, and at least one type of liquid crystal tolan compound including a tolan unit therein.
The chiral nematic liquid crystal composition is preferably regulated by adding a chiral agent to the nematic liquid crystal in an amount sufficient to effectively form a planer structure. The chiral agent is more preferably added to the nematic liquid crystal in an amount sufficient to effectively form a focal-conic structure.
The chiral agent may be included in a range from 10 wt% to 45 wt% relative to the total weight of the nematic liquid crystal composition and chiral agent.
The nematic liquid crystal composition also may include at least one type of liquid crystal tricyclic compound. The inclusion of a liquid crystal tricyclic compound improves the usable temperature range of the chiral nematic liquid crystal composition. The liquid crystal tricyclic compound used may have a cyano group at the terminus. Furthermore, a compound having at least one fluorine atom in its molecular structure may be used.
The total content of the at least one type of liquid crystal ester compound and the at least one type of liquid crystal tolan compound may be 60 wt% or more relative to the nematic liquid crystal composition.
The liquid crystal ester compound included may be 25 wt% or more relative to the nematic liquid crystal composition. The liquid crystal ester compound used will have a cyano group at its terminus.
The liquid crystal tolan compound content may be at least 15 wt% relative to the nematic liquid crystal composition. The liquid crystal tolan compound used has at least one fluorine atom within its molecular structure.
The dielectric anisotropy of the chiral nematic liquid crystal composition is in a range from 9 to 40, and desirably in a range from 15 to 40.
The refractive index anisotropy of the chiral nematic liquid crystal composition is in a range from 0.10 to 0.25, and desirably in a range from 0.14 to 0.22. The chiral nematic liquid crystal composition may include colorant.
According to another aspect of the present invention, a liquid crystal display comprises: a pair of substrates at least one of which is transparent; and a liquid crystal layer retained between said substrates, said liquid crystal layer including the above mentioned chiral nematic liquid crystal composition.